Apparatus of this kind are required notably for magnetic resonance tomography apparatus for medical diagnostics. In the essentially cylindrical central examination space of such apparatus a uniform, steady magnetic field (B.sub.0 field) is generated on which gradient fields are superposed. The field lines of the B.sub.0 field extend in the direction of the cylinder axis of the examination space which is referred to as the z-axis. The steady and/or pulsed gradient fields superposed on the B.sub.0 field cause a deliberate gradient in the steady field distribution in at least one of the coordinate directions x,y,z.
In an object to be examined an RF field which is oriented perpendicularly to the B.sub.0 field and which is as uniform as possible (B.sub.1 field) must be generated with a frequency which corresponds to the Larmor (angular) frequency of the nuclei aligned in the B.sub.0 field. The Larmor frequency of the precessional motion of the spin axis of the nuclei about the direction of the magnetic field is the product of a nucleus specific gyromagnetic factor and the magnetic field strength B.sub.0. The gyromagnetic factor depends on the composition of the nucleus. For the protons of the hydrogen isotope .sup.1 H, for example for B.sub.0 =4 Tesla, a Larmor frequency of approximately 170 MHz is obtained. In magnetic resonance tomography for the examination of patients, notably the MR signals formed by .sup.1 H protons due to the B.sub.1 field are of importance, but the signals of other and different nuclei may also be evaluated for spectroscopic examinations.
An apparatus of the kind set forth is known, for example, from DE-OS 34 10 204. In RF coils and resonators used therein, involving conductor-bound RF currents, the maximum of the B.sub.1 field occurs on the conductor surfaces, a substantially lower field strength existing in the useful space within a so called body coil in which an object to be examined is introduced. The stength of the B.sub.1 field also decreases very quickly towards the interior of the object to be examined in the case of so called surface coils to be arranged on an object to be examined.
The signal-to-noise ratio becomes more favorable for the MR signals to be detected as the strength of the B.sub.1 field generated in the object to be examined increases.